Materials Science and Engineering C 36 (2014) 139–145

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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Pure zinc sulfide quantum dot as highly selective luminescent probe for determination of hazardous cyanide ion Mojtaba Shamsipur a,⁎, Hamid Reza Rajabi b a b

Department of Chemistry, Razi University, Kermanshah, Iran Chemistry Department, Yasouj University, Yasouj 75918-74831, Iran

a r t i c l e

i n f o

Article history: Received 5 October 2013 Received in revised form 16 November 2013 Accepted 2 December 2013 Available online 7 December 2013 Keywords: Pure ZnS Cyanide ion Quantum dot Capping effect Fluorescence quenching Luminescent probe

a b s t r a c t A rapid and simple fluorescence method is presented for selective and sensitive determination of hazardous cyanide ion in aqueous solution based on functionalized zinc sulfide (ZnS) quantum dot (QD) as luminescent prob. The ultra-small ZnS QDs were synthesized using a chemical co-precipitation method in the presence of 2-mercaptoethanol (ME) as an efficient capping agent. The prepared pure ZnS QDs was applied as an optical sensor for determination of cyanide ions in aqueous solutions. ZnS nanoparticles have exhibited a strong fluorescent emission at about 424 nm. The fluorescence intensity of QDs is linearly proportional to the cyanide ion concentration in the range 2.44 × 10−6 to 2.59 × 10−5 M with a detection limit of 1.70 × 10−7 M at pH 11. The designed fluorescent sensor possesses remarkable selectivity for cyanide ion over other anions such as Cl−, − − 2− − − 2− 2− 2− − − Br−, F−, I−, IO− 3 , ClO4 , BrO3 , CO3 , NO2 , NO3 , SO4 , S2O4 , C2O4 , SCN , N3 , citrate and tartarate with negligible influences on the cyanide detection by fluorescence spectroscopy. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The recognition, separation and determination of anions have received considerable attention due to their important roles in biological, industrial, and environmental processes [1,2]. Cyanide ion (CN−) is known as one of the most lethal poisons, and the mechanism of human toxicity for cyanide lies in its ability to suppress the transport of oxygen [3]. CN− ion is widely distributed in the ecosystem and has been associated with toxic effects in humans and animals [4]. Cyanide is also commonly used worldwide in industrial applications such as metal mining, metal plating, and plastics manufacture [5]. Moreover, cyanide played an important role in the cause of death of fire cases, and thus the recognition and determination of cyanide amounts of human blood in fire victims are areas of active research [6]. There are many kinds of qualitative and quantitative methods for analysis of CN− in wide fields, reflecting the great needs of its analysis. Detection of CN− ion can be accomplished by various instrumental methods such as photometry [7], spectrofluorimetry [8], colorimetry [9], GC/MS [10], electrochemical [11], and flow injection [12] techniques. Among the various reported techniques, fluorescent sensors present many appealing advantages, including high sensitivity, low cost, easy detection, and remote control. While a number of synthetic organic luminescent probes for detection of CN− anion have been ⁎ Corresponding author. Tel./fax: +98 741 2242164. E-mail addresses: [email protected] (M. Shamsipur), [email protected] (H.R. Rajabi). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.12.001

designed, a few of them are capable of displaying high selectivity over other anions. In addition, these organic fluorophores suffer from low quantum yield and poor photostability, which thus limits their practical applications [13]. In the recent years, semiconductor nanoparticles or quantum dots (QDs) have been developed as luminescent probes for sensing events [14]. QDs, as a brand new class of fluorescent receptors, have the three dimensions confined to 1–8 nm length scale. QDs possess an excellent photostability, high quantum yields, and long fluorescence lifetimes [15]. In addition, due to the unique and novel properties of the QD based luminescent sensors such as low cost fabrication, reliability, reproducibility, accuracy, high sensitivity and selectivity, expanding applications of QDs to develop inorganic anions, cations, drugs, organic dyes, explosive compounds and biomolecular sensors are a topic of current interest [16–20]. In the case of QDs, zinc sulfide, with wide band gap energy are so far most studied, due to their tunable emission in the visible range, the advances in their preparation, and their potential use for industrial and biomedical applications [21]. Moreover, coating the surface of theses semiconductor nanoparticles with suitable ligands or capping agents has profound effects on the photoluminescence response of the QDs to some chemical species [22]. On the other hand, the surface modification of QDs may change their chemical, optical, and photocatalytic properties. It can give rise to effects, such as (i) an enhancement of their excitonic and defect emission, (ii) an improvement of the photostability of QDs, (iii) the generation of new traps on the QDs surface, leading to the appearance of new emission bands, (iv) an

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enhancement of the selectivity, sensitivity and efficiency of light induced reactions occurring on the surface of QDs, etc. [23]. Mehta et al. have reported the utilization of polyethylene glycol (PEG) capped ZnS nanoparticles (NPs) for cyanide ion detection [24]. PEG capped ZnS NPs were synthesized by microwave process in the range of 12–15 nm. In addition, higher than the amount of ZnS NPs was required for the analyses. In another work, W.J. Jin and co-workers [22] have reported a turnout cyanide sensor based on CdSe NPs functionalized with tert-butyl-N(2-mercaptoethyl)-carbamate groups. This method also has limitations like the use of organic solvent (chloroform) and hazardous CdSe QDs. Because of the use of toxic, volatile organic solvents, the development of a practical and effective cyanide sensor is in infancy. In this work, we have used a very simple, fast and reproducible process for the synthesis of ultra-small ZnS QDs by chemical coprecipitation method. Free-standing clusters of semiconductors can be easily formed in large quantities by relatively inexpensive means in aqueous media, at room temperature. Characterization of the prepared ZnS QDs was carried out with various techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV–vis and fluorescence spectroscopy. In addition, an optical based on the fluorescence quenching of ZnS QDs in the presence of CN− ions was developed for selective determination of trace amounts of CN− ions in aqueous media. 2. Experimental 2.1. Apparatus and materials All fluorescence spectra were recorded on a Cary Eclipse fluorescence spectrophotometer (Varian). The UV–vis absorption spectra of the ZnS QDs were also recorded using an Agilent 8453 UV–Vis spectrophotometer. A digital pH meter, Metrohm model 692, equipped with a combined glass calomel electrode was used for the pH adjustments. The morphology of the prepared ZnS QDs was examined using a JEM200CX transmission electron microscope. The X-ray diffraction pattern was obtained using German Bruker D8Advanced Diffractometer with Cu Kα source (λ = 1.5406 A). Reagent grade sodium sulfide (Na2S·9H2O), 2-mercaptoethanol (ME), and zinc nitrate salt, and other anions used (all from Merck) were of the highest purity available and used without any further purification. All solutions were prepared using double distillated water. 2.2. Preparation of ZnS QDs ZnS QDs were synthesized by a chemical co-precipitation method at room temperature, without using any organic solvent following the previous literature [17]. For this purpose, 50 mL aqueous solution of 0.01 M zinc nitrate was prepared in double distillated water. The solution was transferred to a three-necked flask, under N2 inert gas with continuous magnetic stirring. Then, 50 mL aqueous solution of ME (0.01 M) as capping agent was added drop wise to the above solution under vigorous stirring. Then, 50 mL aqueous solution of 0.01 M sodium sulfide was added drop-wise to the solution. The reaction was completed for 1 h, and the precipitated nanoparticles of ZnS were separated in a centrifuge at 3000–4000 rpm within 10–15 min, and were washed by water. The resulting powders were dried in vacuum at room temperature for use in further studies. 2.3. Fluorescent detection of hazardous CN− anions An aqueous solution of ZnS QD capped with ME was prepared by dispersion of certain amount of ZnS in doubly distillated water. For the quenching studies, 2.5 mL of aqueous solution of phosphate buffer was placed in the quartz cell (4.0 cm × 1.0 cm × 1.0 cm) and 100 μL of ZnS QD with desired concentrations was added, and then titrated

by successive additions of 50 μL portions of 500 μg mL−1 stock solutions of CN− ion and the solution was mixed before any fluorescence measurement. The fluorescence spectra of ZnS QDs in the absence and presence of the CN− anion were recorded at excitation and emission wavelengths of 265 and 423.9 nm, respectively. The scan speed was 1200 nm min− 1 and the band-slits of both excitation and emission were set as 5.0 nm. 3. Results and discussion 3.1. Characterization of the synthesized ZnS QDs 3.1.1. TEM and XRD studies of ZnS QDs The morphology and particle size of the prepared ZnS QDs were studied by the transmission electron microscopy. Fig. 1 indicates the TEM image of the synthesized ZnS nanoparticles capped by ME with almost spherical particles. TEM image shows that the particles are most rounds with the average particle size of 4 nm. However, it is worth noting that only a small percentage of the total particles showed a diameter size greater than 5 nm. Besides, some large particles are also apparent in Fig. 1, indicating aggregation of QDs. These aggregates may be formed at relatively high QD concentrations (most likely during the solvent evaporation process of TEM experiments) via interconnection of the ME capped ZnS QDs to form a three-dimensional ZnS QD network [19,22]. In addition, for investigation of the crystalline structure and purity of nanoparticles, X-ray diffractometry was performed on the ME capped ZnS Nanoparticles. Fig. 1 shows the XRD pattern of a typical ZnS nanoparticle sample. The crystal structure of the sample is cubic zinc blend with peaks indexed as (111), (220) and (311), which match well with the standard card, JCPDS NO 5-566 [25]. The XRD peaks of samples are broad due to the small crystalline domains in the nanoparticles. On the other hand, the average particle size of the ZnS QDs was obtained about 2.1 ± 0.2 nm using the Debye–Scherrer relation [26]. 3.1.2. Optical characteristics of ZnS nanoparticles The optical properties of ME capped ZnS QDs were characterized by UV–Vis absorption spectrometry and fluorescence spectroscopy. The results are shown in Fig. 2(a), wherein QDs exhibited a broad absorption spectrum and narrow emission band with a characteristic band at 271 nm and 423.9 nm, respectively. Quantum confinement effects are expected in the colloids due to small particle size detectable as a blue-shift of nano-phase optical absorption edge caused by band gap widening in the UV region [27] and a corresponding blue-shift in photoluminescence (PL). In the corresponding fluorescence spectrum of ZnS QD, a narrow maximum emission band centered at 423.9 nm was observed when QDs were excited by the radiation of 265 nm. The narrow emission spectrum indicated that high degree of monodispersity of colloidal ZnS QDs is present [28]. The emission band in the PL spectra of ZnS nanocrystals at about 424 nm could be assigned to the radiative recombination involving defect states in the ZnS nanocrystals [29]. As in the bulk ZnS, the dominant defect involving the violet–blue emission in QDs is sulfur vacancies (VS), which form shallow donor levels below the conduction band (CB) [30]. The energy levels of defects VS become deeper in the band gaps of ZnS QDs than in the bulk counterpart because of quantum confinements. The electron-hole pairs created by ionization are possibly trapped at these defects, where they recombine and generate luminescence [14]. The UV–Vis optical spectrum of the synthesized ZnS in the presence of constant concentration of ME as capping agent is shown in Fig. 2. As can be seen, the absorption spectra revealed an absorption edge wavelength at 271 nm. UV–vis spectrum was used for evaluation of an approximate size of prepared ZnS nanoparticles [31]. The method developed by Swanepoel [32] was used to calculate the absorption coefficient α. An expression for the absorption coefficient,

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Fig. 1. Typical (a) TEM image and (b) XRD pattern of ME capped ZnS QDs.

α(hv), as a function of photon energy (hv) for direct and indirect optical transitions is given by the following expression:

α¼

 n A hv−Eg hv

where α is the absorption coefficient, hv is the photon energy, A is a constant, Eg is the optical gap and n is a constant equal to 1/2 for direct gap semiconductors and 2 for indirect gap semiconductors [33]. Plotting (αhv)2 against photon energy (hv) gives a straight line with intercept equal to the optical energy band gap. Fig. 3 shows the change of (ahv)2 as a function of energy. By extrapolating the straight-line portion of the plot to zero absorption coefficient can be estimated the corresponding Eg value of 4.25 eV (at λ = 271 nm), while, the absorption shoulder for macrocrystalline ZnS is seen at about 350 nm. According to Brus effective mass approximation [34], the average size of ZnS was estimated as 2.2 nm, which was consistent with the result of XRD. Moreover, this obvious blue

shift in the absorption wavelength (from 350 to 271 nm) showed a significant quantum confinement effect [35] of synthetic ZnS QDs since quantum confinement effects dominate the optical properties of QDs, and indicate that the size of prepared particles is less than 6 nm [17]. 3.2. Effect of pH It is well known that determination of metal ions and anions using various techniques is pH dependent [36,37]. In order to find the optimum conditions for the determination of CN− ion, the influence of various pH on the luminescence of ZnS QDs was investigated in the range of 4–13. Aqueous phosphate buffer solutions were used to achieve the desired pHs. Fig. 3 shows the effect of the pH on the fluorescence intensity of ZnS QD capped with ME before (Fo) and after (F) addition of CN− ion. The results showed a decrease of the luminescence intensity of ZnS QDs at pH lowers than 6, and the maximum PL intensity was observed at higher pHs (Fig. 3a). In addition, the effect of pH value on the intensity of binary QD–CN− system was also studied (Fig. 3b).

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a

400 B

300

2

200 1 100

Fluorescence Intensity

Absorbance

3

264

314

364

414

464

Wavelength (nm) 2

(ahv)2(eV2cm-2)

b 1.5

1

0.5

0 2.8

3

3.2

3.4

3.6

3.8

4

4.2

4.4

hv (eV) Fig. 2. (a) UV–vis absorption and fluorescence spectra of ME capped ZnS QDs, and (b) the corresponding plot of (αhv)2 vs. (hv).

Fluorescence Intensity

500

a

F0 F

400 300 200 100 0

4

5

6

7

8

9

10

11

12

3.3. Effect of ZnS QDs concentration The influence of the concentration of ZnS QDs on fluorescence quenching by CN− ions was studied, too (Fig. 4). It is well known that the concentration of QDs affects the fluorescence intensity. However, if the concentration was too low, when a given emission slit was settled, the fluorescence intensity was also very low, which may sacrifice the linear range. Due to increase in active sites of QDs, the number of luminophore particles was increased and thus PL intensity of solution was reached to maximum intensity in the presence of 50 μg mL−1 of ZnS QD. However, as the ZnS QD concentration was increased beyond the optimum amount, the PL intensity declined because of increase in the opacity of the solution samples and therefore, the light scattering was multiplied. High concentration of ZnS nanoparticles may result in self-quenching of the QDs fluorescence, too [38]. As shown in Fig. 4, the fluorescent intensity increased with the increase in concentration of ZnS QDs and reached to a maximum of 50 μg mL−1. Later, the PL intensity dropped down with the increase in the concentration of ZnS QDs. Take both into consideration, the QD concentration of 50 μg mL−1 was recommended as an optimum concentration of ZnS QD for determination of cyanide ions in the further investigations.

0

0 214

The fluorescence intensity reached its highest at the pH of 11.0. In the strong acid the interaction between QDs and CN− was weak because CN− existed in the form of HCN instead of CN−. However, in the low value of pH, dissolvation of ZnS QDs is possible, too. At the same time, the response was also weak in strong base medium (pH N 11) because the existence of too much OH− groups on the surface of QDs hindered the interaction between CN− and QDs. Therefore, an optimum pH of 11 was selected for further experiments.

13

3.4. Fluorescence detection of cyanide by the quenching emission of ZnS QDs In order to achieve the sensitive detection of CN−, the quenching study of ZnS QDs by addition of CN− ion was investigated pH 11 of phosphate buffer. Results showed that the PL intensity of ZnS QDs decreased with the increase in incubation time of ZnS QDs with CN− immediately. The quenching effect of CN− with different concentrations of ZnS QDs is shown in Fig. 5(a). As seen, the gradually changes in fluorescent spectrum of ZnS QDs upon addition of CN− ions are obvious. It was found that linear relationship existed between the decrease in PL intensity and successive addition of CN− concentration. Utilizing the intensive quenching effects on ME capped ZnS QDs, optical sensing method of CN− can be established.

pH 500 3

Fluorescence Intensity

b

F0/F

2.5 2 1.5 1 0.5

2

4

6

8

10

12

14

pH

400

300

200

100

0

50

100

150

200 -1

ZnS Concentration (µg mL ) Fig. 3. (a) Effect of pH on the fluorescence intensity of QDs (50 μg mL − 1 ), and (b) the fluorescent intensity change of system containing ZnS QDs (50 μg mL − 1 ) and CN− (1.0 × 10 − 5 mol L − 1 ) in different pH solutions.

Fig. 4. Effect of the concentration of ME capped ZnS QDs on the fluorescence intensity (2.0 mL of phosphate buffer at pH = 11, [CN−] = 1.0 × 10−5 mol L−1).

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500

Table 1 Effect of other anions on the selectivity of the constructed cyanide fluorescent sensor.

Fluorescence Intensity

a 400 300 200 100 0 350

375

400

425

450

475

500

Wavelength (nm)

b

3

143

Anion

Mass ratio of anions to CN− ion

Change of fluorescence intensity (%)

Cl− (as NaCl) Br−(as NaBr) F−(as NaF) I−(as NaI) IO− 3 (as KIO3) BrO− 3 (as NaBrO3) ClO− 4 (as KClO4) CO2− 3 (as Na2CO3) NO− 2 (as NaNO2) NO− 3 (as KNO3) 2− SO4 (as Na2SO4) S2O2− 4 (as Na2S2O4) C2O2− 4 (as Na2 C2O4) SCN−(as KSCN) N− 3 (as NaN3) Citrate (as Na3C3H5O(COO)3·2H2O) Tartarate (as NaKC4H4O6·4H2O)

1000 1000 1000 800 800 800 1000 1000 1000 1000 800 700 800 500 800 1000 1000

+1.7 −0.5 −0.3 +3.7 −1.6 +0.2 +1.1 −0.6 +0.8 +0.2 −1.8 −0.9 −0.6 −3.4 +1.7 +2.2 +1.3

F0/F

2.5 2 1.5 1 0.5

0

5

10

15 -

20

25

30

-6

[CN ]×10

Fig. 5. (a) Effect of CN− concentration (0.00–2.59 × 10−5 mol L−1) on the fluorescence intensity of ZnS QDs and (b) the Stern–Volmer plot of CN− concentration dependence of the fluorescence intensity of QDs with a 0.993 correlation coefficient. Conditions: 2.0 mL of phosphate buffer at pH = 11, 50 μg mL−1 of ZnS QD, at λex = 265 nm.

In a homogeneous medium having only a single component exponential decay, the quencher concentration could be obtained from the well-known Stern–Volmer relationship. Interestingly, the plot of Fo/F versus CN− quencher concentration was fitted by conventional linear Stern–Volmer equation, which Fo and F are the fluorescence intensity of the probe in the absence and presence of cyanide ions. The resulting plot (Fig. 5b) shows a good linear relationship. Stern–Volmer quenching constant (KSV), which is a measurement of the quenching efficiency of the quencher, was calculated to be 7.61 × 104 L mol −1. The

Anion 50 0

F0-F

-50

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18

-100

linear range of the calibration curve was from 2.44 × 10−6 to 2.59 × 10−5 mol L− 1 with the detection limit of 1.70 × 10− 7 mol L−1. The standard deviation for six replicate measurements of a solution containing 5.0 × 10−6 mol L−1 of CN− ion was 2.6%. The linear relationship (R2 = 0.993) of the Stern–Volmer plot of Fo/F versus CN− concentration suggested that a single class of fluorophores was equally accessible to all the quenchers. It was concluded that the fluorescence quenching of QDs upon addition of CN− was ascribed to be the binding action between QDs and CN− [24]. 3.5. Interference study of foreign anions The effect of various anions on the fluorescence spectrum of ZnS QDs was also investigated by fluorescence titration of QDs with various anions. Under optimal experimental condition, in the mixture solution of CN− ion (5.0 × 10−6 mol L−1) and 50 μg mL−1 QDs, the presence of excess amounts of foreign anions compared with the concentration of CN− resulted in less than ±5% error. No obvious fluorescent intensity change upon the addition of competitive ion such as Cl−, Br−, F−, I−, − − − 2− − − 2− 2− 2– − IO− 3 , ClO4 , BrO3 , CO3 , NO2 , NO3 , SO4 , S2O4 , C2O4 , SCN , N3 , citrate, and tartarate anion was observed. The results are presented in Fig. 6. As seen, optical response of the ME capped ZnS QDs towards CN− ion is highly selective and influence of other anions is very weak, even at a relatively higher concentration. As shown in Fig. 6, some an− − − 2− 2− − 2− ions such as: C2O2− 4 , F , IO3 , SO4 , S2O4 , SCN , N3 , CO3 , and citrate have a relative fluorescence enhancement effect on the emission intensity of ZnS QDs at relative high concentration. Moreover, other anions − − − − − − including NO− 2 , NO3 , Cl , I , BrO3 , ClO4 , N3 , and tartarate show weak quenching effect on the fluorescence of functionalized QDs. As can seen from Table 1, anions showed little or no effect on the fluorescence intensity of ZnS QDs even when the concentrations of some anions were 1000 times higher than that of CN− ion. However, fluorescence intensity of ZnS QDs was minimally affected by other anions and was significantly affected by the CN− ion. Therefore, those ions of high

-150 -200 -250 Fig. 6. The change in fluorescence intensity of ZnS QDs (50 μg mL−1, pH = 11 of phosphate buffer) in the presence of various anions. The concentration of CN− anion was 5.0 × 10−6 mol L−1 while the concentrations of other anions were 5.0 × 10−3 mol L−1. − − 2− − − − − − (1) C2O2− 4 , (2) NO3 , (3) NO2 (4) SCN , (5) F , (6) Cl , (7) Br , (8) I , (9) SO4 , (10) − − 2− − − S2O2− , (11)IO , (12) BrO , (13) CO , (14) Citrate, (15) ClO , (16) tartarate, (17)N 4 3 3 3 4 3 , and (18) CN− ions.

Table 2 Determination results of CN− ion in two water samples under optimal conditions (N = 3). Sample

Added (μM)

Found (μM)

Recovery (%)

RSD (%)

Tap water

0.0 3.0 5.0 0.0 3.0 5.0

NDa 2.87 4.84 NDa 3.05 5.12

– 95.7 96.8 – 101.7 102.4

– ±1.82 ±1.55 – ±2.27 ±1.08

Ground water

a

Not detected.

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Table 3 Comparison of the prepared cyanide luminescent probe with some methods reported in literature. Method

pH

DL

LR

RSD (%)

Reference

Spectrophotometry FIA/spectrophotometry Optode/spectrophotometry ISE/potentiometry CPE FIA/FAAS SPME QCM QD/fluorimetry QD/fluorimetry QD/fluorimetry

7 11–12 10 11–13 10 9–11 – 9–11 7 9–11 11

0.33 μM 0.02 μg.mL−1 4.2 × 10−6 M – 9 × 10−6 M 0.06 mg.L−1 4.3 μg.L−1 0.3 mg.L−1 1.5 × 10−7 M 1.1 × 10−6 M 1.7 × 10−7 M

0.0–1.0 μM 0.4–5.2 μg.mL−1 1.0 × 10−5–2.5 × 10−4 1.0 × 10−4–1.0 M 1.5 × 10−5–1.0 × 10−2 0.1–10.0 mg.L−1 0.02–0.5 mg.L−1 0.0–10 mg.L−1 3.0 × 10−7–1.2 × 10−5 0.0–2.5 × 10−4 M 2.4 × 10−6–2.6 × 10−5

– 1.2 2.0 b5 4.57 4.7 3.9 1.7–9.3 2.2 − 2.6

[46] [47] [48] [24] [49] [50] [51] [52] [13] [53] Present sensor

M M

M M

Abbreviations: DL: detection limit, LR: linear range, RSD: relative standard deviation, FIA: flow injection analysis, FAAS: flame atomic absorption spectrometry, CPE: carbon paste electrod, QCM: quartz crystal microbalance, SPME: solid phase microextraction.

concentration couldn't coexist with CN−, and their existence didn't interfere with the fluorescence determination of hazardous CN− anion. 3.6. Determination of cyanide in water samples To test the applicability of the present method, ZnS QDs probe was used to determine CN− in water samples and the results were listed in Table 2. The water samples were treated according to the literature [39,40]. Under the optimized conditions, the recoveries were found to be 95.7–102.4% with RSDs ranged from 1.08 to 3.82% with standard addition method. 3.7. Suggested mechanism Several quenching mechanisms have been reported to explain how ions quench the fluorescence of QDs. Non radiative recombination pathway, inner filter effect, ion binding interaction, and electron transfer process are the possible mechanisms to explain the quenching phenomena [41,42]. However, conversely to typical Stern–Volmer quenching behavior, which is driven by collisions between quencher (CN−) and luminescent molecules (ZnS), the quenching of the luminescence of the QDs is attributed to anion binding followed by a redox reaction on the surface of the QDs. Fluorescent quenching characteristics of this sensor show that there was no significant shift in emission wavelength (emission band centered at 424 nm) with increasing concentration of CN− ion. Therefore, the quenching phenomenon in this system is possibly attributed to the effective electron transfer from QD to CN− ion. This implies that these CN− ions effectively quench the fluorescence of ZnS QDs facilitating nonradiative recombination of excited electrons (e−) in the conduction bands and holes (h+) in the valence band [43]. When CN− reached the surface of the QDs, CN− vacancy of the surface of QDs decreased. As the result, surface fluorescence of ZnS was quenched effectively. At the same time, the adsorption of CN− increased dangling bonds originating from the lone pairs on surface CN− which resulted in more no-radiation pathways of luminescent center, consequently the fluorescence of ZnS was quenched as well [44]. Since the surface of QDs was coated with negatively charged of mercaptoethanolic groups in alkaline pH, it was very difficult for other anions to interact with the surface of the QDs. However CN− could fit well the cyanide defect of the surface and CN− could bind strongly with Zn2+, thus CN− could easily interact with the QDs [45]. Moreover, sodium cyanide can form a layer of zinc complex, as Na2[Zn(CN)4], on the surface of ZnS QDs. Therefore, as a result, it is not too surprising that highly selective response to CN− anion happened. 4. Conclusion In conclusion, we have demonstrated a novel strategy for the selective detection of hazardous cyanide ion by using ZnS QD as luminescent

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